Contact lenses, in particular soft contact lenses for single use, are nowadays produced in large volumes in highly automated manufacturing processes and facilities. These contact lenses can be manufactured using reusable male and female mold halves which are typically made out of glass. When mated to form the mold, these mold halves define a hollow mold cavity between the lens forming surfaces of the male and the female mold half. Prior to mating the male and female mold haves to close the mold, a lens forming material is dosed into the female mold half. Examples of such lens forming materials include polymers and pre-polymers based on polyvinyl alcohols (PVA), silicone hydrogels (SiHy) or on polyethylene glycols (PEG) or other suitable lens forming materials known in the art. After closing the mold, the lens forming material contained in the mold cavity is cured through polymerization and/or cross-linking to form the contact lens.
During manufacturing of such contact lenses, it may occur, that air bubbles get entrapped in the lens forming material resulting in a contact lens which will be rejected during the lens inspection step, as the entrapped air bubbles may lead to a poor quality of the lens. It is believed that the formation of air bubbles may result from a non-uniform and/or non-symmetric wetting of the respective mold half by the lens forming material. For example, as the male mold half is moved towards, the lens forming material contained in the female mold half, in case the first contact of the lens forming surface of the male mold half does not occur at a predefined single point, a non-homogeneous wetting might result with an entrapment of air bubbles.
To overcome this problem of air bubble entrapment, an electrostatic charge is provided onto the lens forming surface on at least one of the mold halves prior to the lens forming surface coming into contact with said lens forming material. The electrostatic charge may be deposited onto the lens forming surface by a charging electrode discharging charges onto the respective lens forming surface or by a triboelectrical stamp carrying an electrical charge and contacting the lens forming surface of the mold half.
This step of applying an electrical charge to the lens forming surface has to be repeated before each lens molding process, in particular after rinsing and drying of the mold halves. For various reasons, the deposition of the electrical charge onto the lens forming surface may not occur properly so that no electrical charge is present on the lens molding surface, or the charge is deposited at a location of the lens mold other than the lens forming surface.
Therefore, a method for reliably allow for contactless detection of an electrical charge on the lens forming surface of a lens mold is needed.